Recently, signatures of high-temperature superconductivity (HTSC) with T_c ≈ 80K were found at pressures above 14GPa in the crystals of La₃Ni₂O₇, which is the n = 2 member of the Ruddlesden-Popper (R-P) nickelate oxides denoted as La_n+1Ni_nO_3n+1 (n = 1, 2, 3, …，∞). This discovery has immediately attracted tremendous research interest as a new family of HTSC. However, subsequent studies revealed that the La₃Ni₂O₇ single crystals grown with the optical-image floating-zone method under moderate oxygen pressure present some sample-quality issues such as the chemical inhomogeneity, oxygen vacancy, and the coexistence of minority monolayer (n=1, 214) and trilayer (n=3, 4310) R-P phases. These complications lead to significant sample-dependent behaviors of existing experiments and the absence of direct experimental evidence for bulk HTSC in the bilayer nickelate La₃Ni₂O₇. Thus, achieving bulk HTSC and identifying the exact phase at play are the most prominent tasks at present.

In light of these challenges, a collaborative team led by Jinguang Cheng from the Institute of Physics, Chinese Academy of Sciences, resorted to the polycrystalline samples of bilayer nickelates La_3-xPr_xNi₂O_7-δ, which can be prepared in a relatively large quantity with better controlled quality and reproducibility via wet-chemistry sol-gel method. They found that the above-mentioned sample-quality issues of La₃Ni₂O₇ can be effectively resolved via substitution of smaller Pr ions for La, leading to the successful synthesis of high-purity La₂PrNi₂O₇ with an ideal bilayer R-P structure (Fig. 1). This sample undergoes a pressure-induced structural transition from orthorhombic Amam to tetragonal I4/mmm at Pc ≈ 11GPa (Fig. 2). Their combined resistivity and ac magnetic susceptibility measurements under hydrostatic pressures provided the key evidence of bulk HTSC, including the zero resistance with high T_c^onset = 82.5K and T_c^zero = 60K and the clear diamagnetic response with a superconducting shielding volume fraction of 97% (Figs. 3, 4). In addition, by employing a suite of experimental techniques, they demonstrate the structural disorders as unfavorable factors for HTSC of La₃Ni₂O₇. These results provide critical experimental evidence for bulk HTSC in the pressurized La₂PrNi₂O₇, confirming the bilayer R-P phase as the source of HTSC and revealing the intergrowths of 327/4310 and 327/214 phases as detrimental factors for bulk HTSC in the La₃Ni₂O_7-δ.

This work not only resolves the existing controversies but also highlights pathways for further exploration of bulk HTSC in bilayer nickelates, providing valuable guidance for the optimization and synthesis of nickel-based high-temperature superconductors.

This study entitled “Bulk High-Temperature Superconductivity in Pressurized Tetragonal La₂PrNi₂O₇” was published on Nature.

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China and the Chinese Academy of Sciences.